Handwritten Captchas: Are They Secure?

TL;DR

Handwritten captchas are generally not a good security measure on their own. While they can deter simple bots, advanced AI and human-based services easily solve them. Use stronger methods like reCAPTCHA v3 or two-factor authentication.

Why Handwritten Captchas Aren’t Very Secure

Handwritten captchas rely on the difficulty computers have with image recognition. However, this gap is closing rapidly. Here’s why they are weak:

• AI advancements: Machine learning models (like those used in Optical Character Recognition – OCR) can now accurately read handwritten text with high success rates.
• Human-based services: “Captcha solving farms” employ people to solve captchas for very little money, bypassing automated checks.
• Accessibility issues: They are difficult for users with visual impairments or motor skill challenges.

Step-by-step Guide: Why You Shouldn’t Rely on Them

1. Understand the limitations of OCR: While basic OCR might struggle, modern OCR engines perform well on clear handwriting.

   # Example using Tesseract OCR (Python)
   import pytesseract
   from PIL import Image
   text = pytesseract.image_to_string(Image.open('captcha.png'))
   print(text)

   This simple example shows how easily you can attempt to read a captcha image.

2. Recognise the threat of AI solvers: Several services offer automated captcha solving using AI. These are readily available and often inexpensive. They work by sending the captcha image to an AI model that returns the solution.
3. Consider human-based attacks: Captcha farms employ real people who solve captchas quickly for a small fee. This is particularly effective against captchas with low complexity or frequent use.
4. Evaluate accessibility concerns: Handwritten captchas pose significant challenges to users with disabilities, potentially violating accessibility standards (like WCAG).

Better Alternatives

Here are more secure options:

• reCAPTCHA v3: This uses a scoring system based on user behaviour. It’s invisible to users and much harder for bots to bypass. You integrate it into your website, and Google provides a score indicating the likelihood of the user being human.
• Two-Factor Authentication (2FA): Requires a second verification method (e.g., code sent to phone or authenticator app). This is significantly more secure than captchas.
• Honeypots: Hidden fields that only bots will fill out. If the field is populated, it’s likely a bot.
• Rate Limiting: Limit the number of requests from a single IP address within a certain timeframe to prevent brute-force attacks.

If You Must Use Captchas (Not Recommended)

1. Use complex, distorted images: If you absolutely need captchas, make them very difficult to read even for humans.
2. Rotate captcha types frequently: Don’t use the same captcha image repeatedly.
3. Combine with other security measures: Never rely on captchas as your sole security method. Use them in conjunction with rate limiting and other techniques.

cyber security Best Practice

Prioritise robust cyber security solutions over relying on easily bypassed methods like handwritten captchas. Regularly review and update your security measures to stay ahead of evolving threats.